1. Field of the Invention
The present invention relates to an optical transmitter with a plurality of transmitter units each emitting signal light with specific wavelengths controlled by adjusting a temperature of a semiconductor laser diode.
2. Related Background Art
Some optical transmitter applied in the wavelength division multiplexing (WDM) communication system installs a plurality of optical transmitter units each emitting signal light whose wavelength is specific to one of the signal channels in the WDM system. Such a transmitter unit often provides a Peltier device to adjust a temperature of the semiconductor laser diode (hereafter denoted as LD) in the target temperature. The United States Patents issued as U.S. Pat. No. 5,515,682 and U.S. Pat. No. 5,920,583, and Japanese Patent Application published as JP-H10-141781 have disclosed a driver circuit to control such Peltier devices. However, when the transmitter provides a plurality of TEC drivers each corresponding to one Peltier device, the size of the transmitter must be large to install all components. Moreover, as shown in the second prior art, JP-H10-335724A, an ordinary Peltier device shows a resistance of about 1Ω and has a limited current to operate it. The maximum current allowable to the ordinary Peltier device is limited from −1 A to 1 A. Accordingly, in the case that the TEC driver down converts the DC power supply of 5 V, or occasionally 3.3 V, to a voltage necessary to control the Peltier device, where the absolute value is 1 ampere in an example above, an unsatisfactory efficiency of the power conversion is available. We can not overlook this situation from a viewpoint of the low power consumption.
Noting the load efficiency of the electronic power of the transmitter, the efficiency increases as the load impedance becomes larger as shown in FIG. 4. This figure shows the efficiency of the transmitter whose configuration obeys those illustrated in the second prior art, JP-H10-141781A, as varying the load impedance. A reduced loss of the driver for the Peltier device is available for the larger load impedance, which is equivalent to the higher efficiency in the driver circuit.